Combination Systems

In addition to stand-alone Radio Frequency drying, Radio Frequency can be combined with conventional drying methods such as convection and conduction to gain even greater improvements in line speed and product quality. There is great synergy with these combination systems where Radio Frequency excels at heating evenly within the material and pushing the moisture to the surface and conventional methods excel at removing moisture from the surface. Whether the Radio Frequency is used as a pre-heat, boost, or finish, or a full Radio Frequency application, drying can be much more effective with a PSC combination system.

At the beginning of a process Radio Frequency heats the material quickly, evenly and helps move the moisture to the surface. The overall time is shortened in the falling rate zone because the whole product has been heated, not just the surface. The even heating avoids a dry outer layer that causes uneven dispersion of sizing and additives in the product. Another application for Radio Frequency preheat is curing processes. The Radio Frequency is very good at quickly heating the product to a consistent temperature, after which conventional methods are very good at maintaining the temperature for a dwell or cure time.

Radio Frequency energy can be added in the middle of a process line to give a Radio Frequency “boost” to the conventional drying process. In this case, the Radio Frequency heats the inside of the product and drives the moisture to the surface where conventional methods are effective. This option has been used in a paperboard line and the Radio Frequency made the downstream steam cylinders more effective by moving the moisture to the surface where the cylinders now contacted a wet product surface rather than a dry, insulating product surface. As with the other combinations of Radio Frequency and conventional drying, the overall drying time is reduced with the use of the Radio Frequency boost.

Another combination of Radio Frequency and conventional drying is using Radio Frequency to do the finish drying. This is used with good thermal insulating materials such as ceramic fiber mat where the dry isolating surface inhibits the drying process. The graph below shows how an Radio Frequency finish can be used to reduce the drying time of the product.

The first three combinations of Radio Frequency and conventional drying use Radio Frequency in part of the overall process cycle with some significant reductions in drying times. It makes sense that another option is to use Radio Frequency and conventional methods simultaneously during the whole process. This offers the largest potential reduction in drying time of all the methods, although it does complicate equipment design. Greenbank Engineering and the Electricity Council of the United Kingdom did a notable study in this area. The results of their work on Air, Radio Frequency Assisted (ARFA) drying is shown below. It shows the significant reduction in drying time with the addition of Radio Frequency to the standard process using 180°C air. Drying times with other hybrid systems have been reduced from hours to minutes in many cases.

The simultaneous use of Radio Frequency and convection has another interesting application for temperature sensitive products. In this case, the Radio Frequency is used to heat and evaporate the water in the product and ambient air (rather than heated air) is used to remove the moisture from the surface and keep the product temperature lower. This combination offers a “gentle” drying where fast drying is needed but the product cannot be exposed to high temperatures.